1. Field of the Invention
The present invention generally relates to optical information recording/reproducing apparatuses, and more particularly to an optical information recording/reproducing apparatus which optically records information on a recording medium and/or optically reproduces the information from the recording medium.
2. Description of the Related Art
An optical disk unit is an example of a unit which uses an optical information recording/reproducing apparatus. The optical disk unit can be used as a storage unit of a file system or the like, and is suited for storing programs and large amounts of data. In such an optical disk unit, it is desirable that an optical system thereof can accurately record and/or reproduce the information, and that the number of parts thereof is minimized so as to reduce the cost of the optical disk unit as a whole.
Various techniques have been proposed to detect a focal error in the optical disk unit. Generally, the astigmatism technique and the Foucault technique are well known. The Foucault technique is sometimes also referred to as the double knife edge technique.
Compared to the astigmatism technique, the Foucault technique is less affected by the external disturbance that occurs when a track on an optical disk is traversed, the birefringence of the optical disk. Accordingly, the mixture of the external disturbance into a focal error signal when the Foucault technique is employed is extremely small compared to the case where the astigmatism technique is employed. In addition, the Foucault technique detects a reflected light beam from the optical disk by a photodetector which is arranged in a vicinity of an image formation point of the optical beam, and for this reason, an abnormal offset is unlikely generated in the focal error signal even if the reflected light beam shifts from an optical axis. Because of these advantageous features obtainable by the Foucault technique, it is desirable to employ the Foucault technique as the focal error detection technique.
First, an example of an optical information recording/reproducing apparatus within a conventional magneto-optic disk unit which employs the Foucault technique will be described with reference to FIG. 1.
In an optical system of the optical information recording/reproducing apparatus shown in FIG. 1, a laser beam which is emitted from a laser diode 201 is formed into a parallel beam having an oval cross section in a collimator lens 202, and is thereafter formed into a light beam having a circular cross section in a true circle correction prism 203. The light beam from the true circle correction prism. 203 is transmitted through a beam splitter 204, reflected by a mirror 205, and is converged on a disk 207 via an objective lens 206. A reflected light beam from the disk 207 enters the beam splitter 204 via the objective lens 206 and the mirror 205, but this time the reflected light beam is reflected by the beam splitter 204 and is directed towards a beam splitter 208. The beam splitter 208 splits the reflected light beam into two light beams, and supplies one light beam to a magneto-optic signal detection system and the other light beam to a servo signal detection system.
The magneto-optic signal detection system includes a Wollaston prism 209, a lens 210 and a 2-part photodetector 211. One of the two light beams output from the beam splitter 208 is input to the 2-part photodetector 211 via the Wollaston prism 209 and the lens 210, and the 2-part photodetector 211 detects the magneto-optic signal, that is, the information signal, based on the input light beam.
The servo signal detection system includes a condenser lens 212, a beam splitter 213, a 2-part photodetector 214, a composite prism 215 and a 4-part photodetector 216. The other of the two light beams output from the beam splitter 208 is input to the 2-part photodetector 214 via the condenser lens 212 and the beam splitter 213 on one hand, and is input to the 4-part photodetector 216 via the composite prism 215 on the other. The 2-part photodetector 214 forms a tracking error detection system in the servo signal detection system, and generates a tracking error signal by obtaining a difference between the outputs of the 2-part photodetector 214 according to the push-pull technique. The composite prism 215 and the 4-part photodetector 216 form a focal error detection system in the servo signal detection system, and generates a focal error signal based on outputs of the 4-part photodetector 216 according to the Foucault technique. A focus servo operation controls the relative positional relationship of the objective lens 206 and the disk 207 based on the focal error signal, so that an in-focus position is located on the disk 207.
Next, a description will be given of the push-pull technique, by referring to FIGS. 2 and 3. FIGS. 2(a), 2(b) and 2(c) show the relative positional relationship of the light beam which is irradiated via the objective lens 206 and the track on the disk 207, and FIGS. 3(a), 3(b) and 3(c) show a spot of the reflected light beam which is formed on the 2-part photodetector 214 in correspondence with FIGS. 2(a), 2(b) and 2(c).
FIG. 2(b) shows a case where the spot of the light beam is positioned at the center of a guide groove 207a of the disk 207. In this case, the spot of the reflected light beam on the 2-part photodetector 214 is formed as shown in FIG. 3(b), and a light intensity distribution b is symmetrical to the right and left. If the outputs of the 2-part photodetector 214 are denoted by A and B, a tracking error signal TES is generated based on the following formula (1). EQU TES=A-B (1)
In this case, the tracking error signal TES is 0.
If the spot of the light beam in FIG. 2(b) shifts to the right as shown in FIG. 2(a), a light intensity distribution a of the reflected light beam becomes unbalanced and the light intensity at the left detector part of the 2-part photodetector 214 becomes larger as shown in FIG. 3(a). For this reason, the tracking error signal TES in this case takes a positive value.
On the other hand, if the spot of the light beam in FIG. 2(b) shifts to the left as shown in FIG. 2(c), a light intensity distribution c of the reflected light beam becomes unbalanced and the light intensity at the right detector part of the 2-part photodetector 214 becomes larger as shown in FIG. 3(c). For this reason, the tracking error signal TES in this case takes a negative value.
Accordingly, if the spot of the light beam on the disk 207 shifts to the right or left with respect to the central position of the guide groove 207a, the tracking error signal TES which is obtained in the above described manner changes to a more positive or negative value. Thus, it is possible to carry out an appropriate tracking control operation based on the tracking error signal TES.
FIG. 4 shows an example of the shapes of the composite prism 215 and the 4-part photodetector 216. The 4-part photodetector 216 includes detector parts 216a, 216b, 216c and 216d. A focal error signal FES is generated from outputs A, B, C and D respectively output from the detector parts 216a, 216b, 216c and 216d of the 4-part photodetector 216, based on the following formula (2). EQU FES=(A-B)+(C-D) (2)
Ideally, the focal error signal FES is 0 in a state where the spot of the light beam is in focus on the disk 207. In this case, the focal error signal FES having an S-curve as shown in FIG. 5 is obtained depending on the distance between the objective lens 206 and the disk 207. In FIG. 5, the ordinate indicates the focal error signal FES, and the abscissa indicates the distance between the objective lens 206 and the disk 207. The origin (0) on the abscissa corresponds to the in-focus position, and the above distance becomes smaller towards the left and larger towards the right in FIG. 5.
FIGS. 6(a), 6(b) and 6(c) show the relative positional relationship of the objective lens 206 and the disk 207. FIG. 6(a) shows a case where the objective lens 206 is close to the disk 207 and the in-focus position is located above the disk 207 in the figure, FIG. 6(b) shows a case where the in-focus position is located on the disk 207, and FIG. 6(c) shows a case where the objective lens 296 is far from the disk 207 and the in-focus position is located between the disk 207 and the objective lens 206 in the figure.
FIGS. 7(a), 7(b) and 7(c) show beam spots on the 4-part photodetector 216 for each relative positional relationship of the objective lens 206 and the disk 207 shown in FIGS. 6(a), 6(b) and 6(c). FIG. 7(a) shows the beam spots for the positional relationship shown in FIG. 6(a), FIG. 7(b) shows the beam spots for the in-focus positional relationship shown in FIG. 6(b), and FIG. 7(c) shows the beam spots for the positional relationship shown in FIG. 6(c). As shown in FIG. 7(b), the beam spots on the 4-part photodetector 216 have oval shapes in the in-focus position, and a division line E of the 4-part photodetector 216 is positioned at the center of each oval beam spot.
However, in the actual disk unit, the distribution of the quantity of the light beam irradiated on the disk 207 may be unbalanced, and errors may exist in the mounting positions of the composite prism 215 and the 4-part photodetector 216.
The light intensity distribution of the light beam which is emitted from the laser diode 201 can generally be approximated by a Gaussian distribution. Hence, if the optical axis of the light beam emitted from the laser diode 201 matches the optical axes of other optical parts, it is possible to obtain a Gaussian distribution in which the center of the light intensity of the light beam input to the objective lens 206 matches the optical axis (point 0) shown in FIG. 8. However, if the light beam emitted from the laser diode 201 is inclined by an angle .theta. in FIG. 1, the center of the light intensity of the light beam input to the objective lens 206 is shifted from the optical axis (point 0) in the Gaussian distribution as indicated by a dotted line in FIG. 8. The "unbalanced distribution" of the light quantity of the light beam irradiated on the disk 207 or "decentering", refers to such a difference between the optical axis and the center of light beam intensity distribution.
On the other hand, the "mounting error" of the composite prism 215, for example, refers to a positional error of the composite prism 215 in a y-direction in FIG. 4. If such a mounting error exists, the composite prism 215 cannot accurately split the incident light beam into two equal light beams. Generally, if the division line E of the composite prism 215 shifts a distance .DELTA.y in the y-direction from the center of the incident light beam, where the division line E extends in the x-direction in FIG. 4, the value of the mounting error can be obtained from [.DELTA.y/(diameter of light beam)].multidot.100 (%).
For this reason, if the quantity of the light beam which is split into two in the composite prism 215 changes and a positional error of the division line E of the 4-part photodetector 216 occurs, a focal offset is generated. The generation of the "focal offset" means that the focal error signal FES described by the formula (2) becomes 0 at a position other than the in-focus position. Thus, according to the conventional Foucault technique, the tolerable margin of the focal error detection system is extremely small with respect to the unbalanced distribution of the quantity of light beam irradiated on the disk 207, the mounting error of the composite prism 215 and the 4-part photodetector 216 and the like. Therefore, there is a problem in that it is extremely difficult to obtain an accurate focal error signal due to the above error factors.